The increased production flow area provided by a horizontal, as compared to a vertical, wellbore has driven an increase in the drilling and completion of horizontal wells. Such wells have long open-hole sections which remain in contact with the drilling fluid for long periods of time in overbalanced conditions, forming a filter cake on the formation and also thereby initiating solids invasion that may induce formation damage. As a result, the selection of drilling fluid has a major effect on minimizing skin development and maximizing oil production.
Performance advantages of oil-based muds (OBM) and synthetic-based muds (SBM), such as higher penetration rates, improved lubricity, shale stability, decreased fluid loss, and thinner filter-cake characteristics, give these systems advantages in reservoir-drilling applications over water-based mud systems. Furthermore, fluid losses to formation from oil based muds are less damaging since the base is composed of oil rather than water.
Most OBMs and SBMs are invert emulsions composed of droplets of aqueous fluid surrounded by a continuous oil phase. OBM and SBM filter cakes are composed of colloid particles and water droplets dispersed in an oil phase. Such filter cakes are hydrophobic and exhibit a permeability which is lower than the permeability of the rock.
After drilling is complete, a cleanup treatment to remove the OB/SB filter cake from the formation face is needed in order to minimize skin and formation damage, increase production flow and restore the productive zone to a near-natural state. Such cleanup treatments are needed in order to break down the interfacial rheological properties of the filter cake, wash the damaged zone of the wellbore and restore the formation's fluid transfer properties.
Historically, solvent- or aqueous-based systems have been used to remove oil based and synthetic oil based filter cakes. Typically, aqueous surfactant based systems are selected over solvent treatments as mud filter cake cleanup treatments for breaking the emulsion inside the cake and complete phase separation. However, such aqueous surfactant based treatment often creates additional damage by forming an emulsion block in the wellbore due to water saturation. Such emulsion blocks have the potential to block oil production. Further, such systems are either not biodegradable or are less efficacious than desired.
In order to meet more challenging, drilling applications such as for use in deepwater and high-temperature, high-pressure (HTHP) applications, and further to meet stricter health, safety, and environmental standards, new systems to remove OB and SBM filter cakes have been sought. In particular, there is a need for new systems that do not cause the problems associated with the aqueous systems of the prior art and which further are biodegradable.